A battery, such as a lithium-ion secondary battery, has a wound electrode body that is housed in a case (battery case, outer case). A pouring hole for pouring electrolyte is formed to the case of such a battery. After electrolyte is poured in, the pouring hole is closed. This closure of the pouring hole is accomplished, for example, by covering with a sealing cap, and joining the circumferential edge of the sealing cap to the case by welding or the like. Structures in which a pouring hole or other such opening (hole, aperture) formed in a case is closed off by a sealing cap have been disclosed, for example, in Patent Document 1 and Patent Document 2.
Patent Document 1 discloses a structure in which a rubber plug is installed at the upper part of the pouring hole, after which a lid is closed that has in its center a protrusion which mates with the rubber plug and has a flange around its periphery, and the flange of the lid is irradiated with a laser beam to seal the pouring hole.
Patent Document 2 discloses a structure comprising a sealing cap in which a reinforcing component that is stiffer than a thin plate component is provided to the periphery of the thin plate component. In this publication, the sealing cap functions as a safety valve, in which the thin plate component breaks when the pressure inside the case goes over a specific level.
Patent Document 1: Japanese Laid-Open Patent Application 2000-21437
Patent Document 2: Japanese Laid-Open Patent Application 2003-187760
Such batteries are frequently provided with a safety valve that opens up when the interior of the battery goes over a specific pressure, in order to prevent rupture in the event that an abnormal amount of gas is generated in the interior. In this case, the joint strength of the sealing cap needs to be high enough not to be broken by a pressure lower than the pressure at which the safety valve opens.
Also, since the battery case is sealed, a pressure differential between the inside and outside of the case may repeatedly exert pressure on the sealing cap. For example, if the temperature inside the battery case rises high, the pressure inside the case is raised due to the expansion of the air inside the case, thereby the pressure inside the case is exerted that pushes the sealing cap from the inside of the case toward the outside. When the air pressure outside the case drops, the pressure inside the case is similarly exerted that pushes the sealing cap from the inside of the case toward the outside. The above-mentioned lithium ion secondary batteries are installed in hybrid vehicles or electric automobiles, where they may be used in cold climes. When used in a cold clime, the vehicle is started in a state in which it has been cooled by the outside air, but the battery emits heat through charging and discharging while the car is running. Therefore, the temperature of the usage environment can vary widely. In this case, since the battery case is sealed, there is also fluctuation of the pressure exerted on the case by the pressure differential inside and outside the case, and the sealing cap is subjected to a load by the pressure differential inside and outside the case. Over the years, the temperature of the environment in which the battery is used varies repeatedly, so repeated load may act on the joint of the sealing cap. Accordingly, the sealing cap joint needs to be strong enough to have the required durability with respect to this repeated load.
Also, the sealing cap is joined to the case by welding, for example, but this can lead to a certain amount of variance in the weld strength caused by variance in the welding conditions. Consequently, the sealing cap joining step, joining conditions, and so forth need to be managed carefully so as to obtain the required joint strength, factoring in a certain margin of safety.